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D. GENERAL INFORMATION ON THE DESIGN OF HVAC SYSTEMS

D. GENERAL INFORMATION ON THE DESIGN OF HVAC SYSTEMS

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SOUND

with the silencer. For some applications,
acoustically lined sound plenums may be used
in the place of duct silencers.
5. Fan-powered mixing boxes associated with
variable-volume air distribution systems
should not be placed over or near noise-sensitive areas.
6. Air flowing by or through elbows or duct branch
take-offs generate turbulence. To minimize the
flow noise associated with this turbulence,
whenever possible, elbows and duct branch
take-offs should be located at least four to five
duct diameters from each other. For high velocity systems, it may be necessary to increase this distance to up to ten duct diameters in critical noise areas.
7. Near critical noise areas, it may be desirable
to expand the duct cross-section area to keep
the air flow velocity in the duct as low as possible. This will reduce potential flow noise associated with turbulence in these areas.
8. Turning vanes should be used in large 90 degree rectangular elbows. This provides a
smoother transition in which the air can
change flow direction, thus reducing turbulence.
9. Grilles, diffusers and registers should be
placed as far as possible from elbows and
branch take-offs.
10. Dampers in grilles, diffusers and registers
should not be used for balancing.
Table 14-37 lists several common sound sources associated with mechanical equipment noise. Anticipated sound transmission paths and recommended
noise reduction methods are also listed in the table.
Airborne and/or structure-borne noise can follow any
or all of the transmission paths associated with a
specified sound source.
With respect to the quality of sound associated with
HVAC system noise in an occupied space, fan noise
generally contributes to the sound levels in the 63 Hz
through 250 Hz 1/1 octave frequency bands. This is
shown in Figure 11-6 as curve A. Diffuser noise usually contributes to the overall HVAC noise in the 250
Hz through 8,000 Hz 1/1 octave frequency bands.
This is shown as curve B in Figure 11-6. The overall
sound pressure levels associated with both the fan
and diffuser noise is shown as curve D. The RC level

11.10

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VIBRATION

of the overall noise is RC 36. The RC 36 curve is
superimposed over curve D. As can be seen by comparing the RC curve with curve D, the classification
of the overall noise is neutral. Curve D represents
what would be considered acceptable and desirable
1/1 octave band sound pressure levels in many occupied spaces.
In order to effectively deal with each of the different
sound sources and related sound paths associated
with a HVAC system, the following design procedures
are suggested:
1. Determine the design goal for HVAC system
noise for each critical area according to its use
and construction. Use Table 14-35 to specify
the desirable NC or RC levels.
2. Relative to equipment that radiates sound directly into a room, select equipment that will
be quiet enough to meet the desired design
goal.
3. If central or roof-mounted mechanical equipment is used, complete an initial design and
layout of the HVAC system, using acoustical
treatment where it appears appropriate.
4. Starting at the fan, appropriately add the
sound attenuations and sound power levels
associated with the central fan(s), fan-powered mixing units (if used), and duct elements
between the central fan(s) and the room of
interest to determine the corresponding sound
pressure levels in the room. Be sure to investigate the supply and return air paths. Investigate possible duct sound breakout when
central fans are adjacent to the room of interest or roof-mounted fans are above the room
of interest.
5. If the mechanical equipment room is adjacent
to the room of interest, determine the sound
pressure levels in the room associated with
sound transmitted through the mechanical
equipment room wall.
6. Add the sound pressure levels in the room of
interest that are associated with all of the
sound paths between the mechanical equipment room or roof-mounted unit and the room
of interest.
7. Determine the corresponding NC or RC level
associated with the calculated total sound
pressure levels in the room of interest.
8. If the NC or RC level exceeds the design goal,

CHAPTER 11

1/1 Octave Bond Center Frequency - Hz

Figure 11-6 ILLUSTRATION OF WELL-BALANCED HVAC SOUND SPECTRUM FOR OCCUPIED SPACES

9.

10.
11.
12.
13.

14.

determine the 1/1 octave frequency bands in
which the corresponding sound pressure levels are exceeded and the sound paths that are
associated with these 1/1 octave frequency
bands.
Redesign the system, adding additional sound
attenuation to the paths which contribute to the
excessive sound pressure levels in the room
of interest.
Repeat Steps 4 through 9 until the desired
design goal is achieved.
Steps 3 through 10 must be repeated for every
room that is to be analyzed.
Make sure that noise radiated by outdoor
equipment will not disturb adjacent properties.
With respect to outdoor equipment, use barriers when noise associated with the equipment will disturb adjacent properties.
If mechanical equipment is located on upper

floors or is roof-mounted, vibration isolate all
reciprocating and rotating equipment. It may
be necessary to vibration isolate mechanical
equipment that is located in the basement of
a building.
15. If possible, use flexible connectors between
rotating and reciprocating equipment and
pipes and ducts that are connected to the
equipment.
16. If it is not possible to use flexible connectors
between rotating and reciprocating equipment
and pipes and ducts connected to the equipment, use spring or neoprene hangers to vibration isolate the ducts and pipes within the first
twenty feet of the equipment.
17. Use either spring or neoprene hangers. Do not
use both.
18. Use flexible conduit between rigid electrical
conduit and reciprocating and rotating equipment.

11.11

SOUND

E

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where the point of fan operation is other than the point
of peak efficiency. Values for C are obtained from
Table 14-40.

FANS

The sound power generation of a given fan performing
a specific task is best obtained from the fan manufacturers test data. Manufacturers' test data should
be obtained from either AMCA Standard 300-85,
Reverberant Room Method for Sound Testing of
Fans, or ANSI/ASHRAE Standard 68-1986/ANSI/
AMCA Standard 330-86, Laboratory Method of Testing In-Duct Sound Power Measurement Procedure
for Fans. When such data are not available, the 1/1
octave band sound power levels for various fans can
be estimated by the procedures outlined below.
While the size divisions of the fans shown in Table
14-38 are somewhat arbitrary, these divisions are
practical for estimating fan noise. Fans generate a
tone at the blade passage frequency. To account for
this, the sound power level in the 1/1 octave band in
which the blade passage frequency occurs is increased by a specified amount. The number of decibels to be added to this 1/1 octave band is called the
blade frequency increment (Bf). Table 14-39 gives an
estimate of the 1/1 octave band for different types of
fans in which the blade passage frequency occurs
and the corresponding blade frequency increment.
For a more accurate estimate of the blade passage
frequency, B,, the following equation can be used:
Equation 11-11

Example 11-5
A forward curved fan supplies 10,000 cfm of air at a
static pressure of 1.5 in. w.g. It has 24 blades and
operates at 1,175 rpm. The fan has a peak efficiency
of 85%. The fan horsepower is 3 HP Determine the
outlet fan sound power levels.

Solution

From Table 14-40, the correction for off peak efficiency operation is 0 dB. Thus,

where RPM is the rotational speed of the fan in revolutions per minute.
The specific sound power levels associated with fan
total sound power given in Table 14-38 in Chapter 14
are for fans operating at a point of operation where
the volume flow rate equals 1 cfm (0.5 I/s) and the
static pressure is 1 in. wg. (250 Pa). Equation 11-12
is used to calculate the fan total sound power levels
corresponding to a specific point of operation.
Equation 11-12

where Lw is the estimated sound power level of the
fan in dB; Kw is the specific sound power level in dB
from Table 14-38; Q is the flow rate in cfm; Q, is 1
cfm, P is the pressure drop in inches w.g.; P, is 1 in.
w.g., C is the correction factor in dB for the case

11.12

470 Hz is in the 500 Hz 1/1 octave frequency band.
From Table 14-40, the blade frequency increment is
2 dB.
The results are tabulated below. For metric units,
convert the metric data to its U.S. unit equivalents
and calculate as above, using the equivalents in
Chapter 14, Section F

CHAPTER 11

F AERODYNAMIC
NOISE

Equation 11-13
Lw(fo) = K + 10 log,, [63] + 50 log10 [Uc]
+ 10 log,, [S] + 10 log,o [DH]
where fo is the 1/1 octave band center frequency (Hz),
Uc is the flow velocity (ft/sec) in the constricted part
of the flow field determined according to Equation 1116, S is the cross-section area (sq. ft.) of the duct,
DH is the duct height (ft) normal to the damper axis,
and KD is the characteristic spectrum (Figure 11-7).
Figure 11-8 shows a schematic of a single-blade
damper. The regenerated sound power levels associated with dampers are obtained as follows:
Step 1: Determine the total pressure loss coefficient,
C.
Equation 11-14
AP

Aerodynamic noise is generated when airflow in the
duct becomes turbulent as it passes through sharp
bends, sudden enlargements or contractions, and
most devices that cause substantial pressure drops.
Aerodynamic noise is usually of no importance when
the velocity of airflow is below 2000 feet per minute
(10 m/s) in the main ducts; below 1500 fpm (75 m/s)
in branch ducts; and below 800 fpm (4 m/s) in ducts
serving room terminal devices. When the duct system
velocities are in excess of the above or when the duct
does not follow good airflow design principles, aerodynamic noise can become a major problem.
Aerodynamic noise is predominantly low frequency in
spectrum (31.5 through 500 Hz 1/1 octave band center frequencies). Low frequency energy is transmitted
readily, with little loss, through the light gauge walls
of ducts and through suspended acoustic ceilings.
The duct elements covered in this section include:
dampers, elbows with turning vanes, elbows without
turning vanes, junctions, and 90 degree branch take-

C = 15.9 x 106

-

(Q/S)2

where Q is the volume flow rate (cfm), AP is the total
pressure loss (inches w.g.) across the damper, and
S is the duct cross-section area (sq. ft.).
Step 2: Determine the blockage factor, BF.
For multi-blade dampers:
Equation 11-15a

offs.

1. Dampers
BF =

The 1/1 octave band sound power level of the noise
generated by single or multi-blade dampers can be
predicted by Equation 11-13.

-20 I II

IIII

I

I

C -1)
(C - 1)

If C = 1, then BF = 0.50.

For single-blade dampers:

II

I III

I

I

I I I I III

-30

-40
-50
-60
-70
-80
-90
0.2

I I
0.5

111 I
1

I
2

5

I
10

STROUHAL NUMBER,

I I
20

50

111
100

200

St

Figure 11-7 CHARACTERISTIC SPECTRUM, KD, FOR DAMPERS

11.13

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Solution
From the given data: Q = 4,000 cfm; P = 0.5 inches
w.g.; S = 1 sq. ft.; DH = 1 ft.

Step 1: Total pressure loss coefficient, C.

Step 2: Blockage factor, BF

Figure 11-8 DAMPER
Step 3: Constricted flow velocity, Uc.
Equation 11-15b
The results are tabulated below.

Step 3: Determine the flow velocity, Uc (ft/sec), in the
damper constriction.
Equation 11-16

Step 4: Determine the Strouhal number, S,. The
Strouhal number which corresponds to the 1/1 octave
band center frequencies is given by
Equation 11-17

Determine the Characteristic Spectrum, KD.
The characteristic spectrum is the same for all dampers and duct sizes if plotted as a function of the
Strouhal frequency. The characteristic spectrum, KD,
is obtained from Figure 11-11 or from
Equation 11-18

2. Elbows Fitted With Turning
Vanes
The 1/1 octave band sound power levels associated
with the noise generated by elbows fitted with turning
vanes can be predicted if the total pressure drop
across the blades is known or can be estimated. The
method that is presented applies to any elbow that
has an angle between 60 degrees and 120 degrees.
The 1/1 octave band sound power levels generated
by elbows with turning vanes is given by
Equation 11-19

All the required information is now available for calculating the 1/1 octave band sound power levels predicted by Equation 11-13.

Example 11-6
Determine the 1/1 octave band sound power levels
associated with a multi-blade damper positioned in a
12 in. x 12 in. duct. The pressure drop across the
damper is 0.5 in. w.g. and the volume flow rate in the
duct is 4,000 cfm.

11.14

where fo is the 1/1 octave band center frequency (Hz),
Uc is the flow velocity (ft/sec) in the constricted part
of the flow field between the blades determined from
Equation 11-22, S is the cross-section area (sq. ft.)
of the duct, CD is the cord length (in.) of a typical
vane, n is the number of turning vanes, and KT is the
characteristic spectrum (Figure 11-9). In addition to

CHAPTER 11

STROUHAL NUMBER, St

Figure 11-9 CHARACTERISTIC SPECTRUM, K, FOR ELBOWS FITTED WITH TURNING VANES

the above parameters, it is also necessary to know
the duct height DH (ft) normal to the turning vane
length (Figure 11-10). The regenerated sound power
levels associated with elbows with turning vanes are
obtained as follows:
Step 1: Determine the total pressure loss coefficient,
C using Equation 11-14:

Step 4: Determine the Strouhal number, St using
Equation 11-17:

Step 2: Determine the blockage factor, BF using
Equation 11-15a:

Step 5: Determine the characteristic spectrum, KT.
Equation 11-20

Step 3: Determine the flow velocity, Uc (ft/sec), in the
turning vane constriction using Equation 11-16:

The characteristic spectrum is the same for any elbow fitted with turning vanes if plotted as a function
of the Strouhal number. The characteristic spectrum
is obtained from Figure 11-9.
All the required information is now available for calculating the 1/1 octave band sound power levels predicted by Equation 11-19.

Example 11-7

Figure 11-10 900 ELBOW WITH TURNING VANES

A 90° elbow of a 20 in. x 20 in. duct is fitted with 5
turning vanes that have a cord length of 79 inches.
The volume flow rate is 8,500 cfm and the corresponding pressure loss across the turning vanes is
0.16 inch in. w.g. Determine the resulting 1/1 octave
band sound power levels.

11.15

SOUND

Solution
From the given data: Q = 8,500 cfm; AP = 0.16 inch
in. w.g.; S = 2.78 sq. ft.; DH = 1.64 ft; CD = 7.9
inches; n = 5.

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VIBRATION

branch duct associated with air flowing in duct turns
and junctions. Equation 11-21 applies to 90 degree
elbows without turning vanes, X-junctions, T-junctions, and 90 degree branch takeoffs (Figure 11-11).

Step 1: Total pressure loss coefficient, C

Equation 11-21

Step 2: Blockage factor, BF

Equation 11-22

Step 3: Constricted flow velocity, Uc

The results are tabulated below.

where fo is the 1/1 octave band center frequency (Hz),
DB is the equivalent diameter (ft) of the branch duct,
UB is the flow velocity (ft/sec) in the branch duct, SB
is the cross-section area (sq. ft.) of the branch duct,
and KJ is the characteristic spectrum (Figure 11-12).
If the branch duct is circular, DB is the duct diameter.
If the branch duct is rectangular, DB is obtained from
Equation 11-23

The corresponding flow velocity (ft/sec), UB, is given

by
Equation 11-24

3. Junctions and Turns
Equation 11-21 has been developed as a means to
predict the regenerated sound power levels in a

where Q8 is the volume flow rate (cfm) in the branch.
DM (ft) and UM (ft/sec) for the main duct are obtained
in a manner similar to those implied by Equations 1123 and 11-24.

Figure 11-11 ELBOWS, JUNCTIONS, AND BRANCH TAKEOFFS

11.16

CHAPTER 11

STROUHAL NUMBER, St

Figure 11-12 CHARACTERISTICS SPECTRUM, Kj,FOR JUNCTIONS

In Equation 11-21, Ar is the correction term that quantifies the effect of the size of the radius of the bend
or elbow associated with the turn or junction. r is
obtained from Figure 11-13(a) or from
Equation 11-25

Equation 11-28
AT = -1.667 + 1.8 m - 0.133 m2
where m is the velocity ratio that is specified by

where RD is the rounding parameter and S, is the
Strouhal number. RD is specified by
Equation 11-26

Um is the flow velocity in the main duct before the
turn or junction and UB is the flow velocity in the
branch duct after the turn or junction.
The characteristic spectrum, Kj, in Equation 11-30 is
obtained from Figure 11-12 or from

where R is the radius (in) of the bend or elbow associated with the turn or junction and DB is defined
above. The Strouhal number is given by
Equation 11-27

Equation 11-30

In Equation 11-21, AT is a correction factor for upstream turbulence. This correction is only applied
when there are dampers, elbows or branch takeoffs
upstream within five main duct diameters of the turn
or junction being examined. AT is obtained from Figure 11-13(b) or from

Equation 11-29

The regenerated sound power levels in a branch duct
and the continuation of the main duct that are associated with a turn or junction are obtained as follows:
Step 1: Obtain or determine the values of DB and DM.
Step 2: Determine the values of UB and UM.
Step 3: Determine the ratios, DM/DB and m.

11.17

SOUND

AND VIBRATION

Figure 11-13 CORRECTION FACTORS FOR CORNER ROUNDING AND FOR UPSTREAM TURBULENCE

Step
Step
Step
Step

4:
5:
6:
7:

Determine the rounding parameter, RD.
Determine the Strouhal number, S,.
Determine the value of Ar.
If turbulence is present, determine the value

of At.

Step 8: Determine the characteristic spectrum, Kj.
Step 9: Determine the value of the branch sound
power levels, Lw(fo)b.

Step 10: Specify the type of junction and determine
the main duct sound power levels, Lw(fo)m, using
Equations 11-31, 11-32, 11-33, or 11-34.
Equation 11-31
X-Junction:

Example 11-8
Determine the regenerated sound power levels associated with a X-junction that exist in the branch and
main ducts given the following information:
Main Duct: Rectangular-12 in. x 36 in., Volume
flow rate-12,000 cfm
Branch Duct: Rectangular-10 in. x 10 in., Volume
flow rate-1,200 cfm
Radius of bend or elbow: 0.0
No dampers, elbows or branch takeoffs are within
five main duct diameters of junction.

Solution
Step 1: Determine the values of DB and DM:

Step 2: Determine the values of UB and UM:

11.18

CHAPTER 11

Step 3: Determine the ratios, Dm/DB and m:

Step 3: Determine the ratios, Dm/DB and m:

Step 4: Determine the rounding parameter, RD:

Step 4: Determine the rounding parameter, RD:

The results are tabulated below.

The results are tabulated below.

Example 11-10
Example 11-9
Determine the regenerated sound power levels associated with a T-junction that exist in the branch and
main ducts given the following information:
Main Duct: Rectangular-12 in. x 36 in., Volume
flow rate-12,000 CFM
Branch Duct: Rectangular-12 in. x 18 in., Volume
flow rate-6,000 CFM
Radius of bend or elbow: 0.0 in.
No dampers, elbows or branch takeoffs within five
main duct diameters of junction.
Step 1: Determine the values of DB and DM:

Step 2: Determine the values of UB and UM:

Determine the regenerated sound power levels associated with a 900 elbow without turning vanes given
the following information:
Main Duct: Rectangular-12 in. x 36 in., Volume
flow rate-12,000 CFM
Branch Duct: Rectangular-12 in. x 36 in., Volume
flow rate-12,000 CFM
Radius of bend or elbow:-0.0 in.
No dampers, elbows or branch takeoffs within five
main duct diameters of elbow.

Solution
Step 1: Determine the values of DB and DM:

Step 2: Determine the values of UB and UM:

11.19

SOUND

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Step 3: Determine the ratios, DM/DB and m:
Step 3: Determine the ratios, Dm/DB and m:

Step 4: Determine the rounding parameter, RD:
Step 4: Determine the rounding parameter, RD:
The results are tabulated below.
The results are tabulated below.

Example 11-11
Determine the regenerated sound power levels associated with a 90° branch takeoff that exist in the
branch and main ducts given the following information:
Main Duct: Rectangular-12 in. x 36 in., Volume
flow rate-12,000 CFM
Branch Duct: Rectangular-10 in. x 10 in., Volume flow rate-1,200 CFM
Radius of bend or elbow: 0.0 in.
No dampers, elbows or branch takeoffs within five
main duct diameters of takeoff.

GDUCT
TERMINAL
DEVICES
Pressure reducing valves in mixing and variable volume boxes usually have published noise ratings indicating the sound power levels that are discharged
from the low pressure end of the box. The manufacturer may also indicate the requirements, if any, for
the sound attenuation materials to be installed in the

low pressure duct between the box and outlet.
Solution
Step 1: Determine the values of DB and DM:

Step 2: Determine the values of UB and UM:

Some of the box manufacturers also test the noise
radiated from the exterior of the box, however this
data is not usually published. If the box is located
away from critical areas (such as in a storeroom or
corridor), the noise radiating from the box may be of
no concern. If, however, the box is located above a
critical space and separated from the space by a
suspended acoustical ceiling which has little or no
transmission loss at low frequencies, the noise radiated from the box may exceed the noise criterion
for the room below. For this case it may be necessary